This application is based on application No. H11-057982 filed in Japan on Mar. 5, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a projector optical system, and more particularly, to a projector optical system provided with a Digital Micromirror Device(trademark) (DMD(trademark), manufactured by Texas Instruments Incorporated. Hereinafter, referred to simply as digital mirror device or DMD) having a multiplicity of variable light reflection angle mirror devices capable of reflecting only signal light in the direction of the projection optical system by changing the angle of reflection of the emitted light according to video signals.
2. Description of the Prior Art
In recent years, with increasing demands for higher-definition images, development of a technology to greatly increase the number of pixels without increasing the size of the optical system has been required in the field of the projector. In view of this requirement, a projector using a DMD has been developed.
In the DMD, minute rectangular mirror devices of high reflectance capable of changing the angles of rotation according to video signals are formed on a silicon memory chip by use of a semiconductor technology. The projector using the DMD controls the direction of reflection of the illuminating light by changing the angles of rotation of the mirror devices and converges only desired reflected light on the screen, thereby enabling projection of a desired image.
FIGS. 7A and 7B show the principle of the DMD in the projector optical system. The manner of travel of light when the mirror devices of a DMD 101 are in an ON state is shown in FIG. 7A. The manner of travel of light when the mirror devices are in an OFF state is shown in FIG. 7B. As shown in FIG. 7A, the illuminating light incident, through an input/output separating prism system 102, on devices 101a of the DMD 101 which are in the ON state is reflected substantially in the direction of the optical axis Ax of a projection optical system 103 (hereinafter, this reflected light will be referred to as ON light). The ON light is projected onto the screen by the projection optical system 103. The optical path of the light directed to the projection optical system 103 to be projected onto the screen will hereinafter be referred to as effective optical path.
The input/output separating prism system 102 separates the input/output light by using the difference in angle of incidence on a surface 102a, that is, the illuminating light is reflected by the surface 102a and directed the DMD 101, and the light reflected from the DMD 101 is transmitted by the surface 102a. 
The illuminating light incident on devices 101b in the OFF state having a different angle of rotation from the devices 101a in the ON state is reflected in a different direction from the ON light as shown in FIG. 7B (hereinafter, this light will be referred to as OFF light).
However, in the conventional projector optical system, there are cases where the OFF light as shown in FIG. 7B is incident on the projection optical system. Such light follows the effective optical path to form an image (ghost) different from the image that should be formed on the screen or to decrease the contrast of the projection screen.
FIG. 8 shows an example in which the OFF light is incident on the projection optical system. The manner of travel of the OFF light from positions P21, P22, P23 and P24 of the DMD 101 is shown in FIG. 8. The projection optical system 103 is disposed so that the ON light from the mirror devices is incident thereon. However, the OFF light ray 105 from the position P21 on the left and the ON light ray 104 from the position P24 on the right are not completely separated at the position of the projection optical system 103, so that the projection optical system 103 taking in the ON light ray 104 simultaneously takes in the OFF light ray 105.
FIG. 9 shows a conventional example different from that shown in FIG. 8. In this example, to solve the problem of the optical system of FIG. 8, the distance between the projection optical system 103 and the DMD 101 is set longer. In this optical system, since the OFF light ray 105 from the position P21 and the ON light ray 104 from the position P24 are completely separated at the position of the projection optical system 103, the OFF light is never incident on the projection optical system 103. However, such a structure results in an increase in the size of the optical system.
An object of the present invention is to provide a projector optical system that is small and capable of producing high-contrast and ghost-free projection images.
To achieve the above-mentioned object, according to one aspect of the present invention, an optical apparatus is provided with: a reflective type spatial light modulator for separating incident light by reflecting it in a first direction and a second direction different from each other; an optical function surface reflecting a reflected light ray in the first direction and transmitting a reflected light ray in the second direction; and an optical system on which one of the reflected light rays in the first and the second directions exiting from the optical function surface is incident.
According to another aspect of the present invention, a projector is provided with: a light source; an illuminating optical system from which light from the light source exits as illuminating light; a reflective type spatial light modulator for reflecting the illuminating light from the illuminating optical system and separating it into ON light and OFF light that are reflected in directions different from each other; a prism having an optical function surface transmitting one of the ON light and the OFF light reflected from the modulator and reflecting the other one of the ON light and the OFF light; and a projection optical system for projecting the ON light exiting from the optical function surface.